I renew my contract every Nov. 1st as playfelt mentioned. It's because I'm still learning so much every year and I always have changes and updates and write a letter to all the parents a couple months before the contract renews to warn them of any major changes and ask them to talk to me ahead of time if they have any issues with my changes. So far nobody has every questioned my policies or changes.

Be fair and give the families warning and you will never have any problems when they respect and appreciate you as part of the team raising their child.